The guards say they were promised an $8,000 bonus for completing a 12-month stint working on Manus Island, despite G4S knowing at the time the company had lost the contract to Transfield Services/Wilson Security and it would be impossible for staff to earn the bonus.

A G4S employee told the ABC some people gave up long-term jobs to join G4S, thinking their employment would be ongoing.

The employee spoke on condition of anonymity for fear of losing their job.

A website called Manus Class Action has been established and a linked Twitter account says it went live on March 16.

The website says potential issues include termination of contracts, bonuses, recruitment, duty of care to staff and entitlements for injuries.

"This site is not suggesting that we do or do not have a valid case to present," the Manus Class Action website said.

"It is however, the only real opportunity to gather as a group, to seek advice and force the matter before the appropriate forum for decision, should we continue to be mislead or uninformed of our futures."

The ABC has been told approximately 100 G4S employees or former employees are in discussions about legal action, but cannot confirm that information.

The company declined to address the specific claim about bonuses, but did email the ABC with a response to the prospect of a class action.

"G4S is not aware of any current or former employees pursuing legal action with respect to aspects of their contracts as stated in your questions," wrote a G4S spokesman on March 26.

G4S was in charge of security when violent riots occurred at the centre on February 16-18 and are due to hand over operations to Wilson Security today.

Expatriate guards worked with locally-hired Papua New Guinean guards, who have been blamed by witnesses for much of the violence.

In the days after the violent riots that left Iranian Reza Barati dead, several expatriate G4S guards left Manus Island, telling the ABC they had quit or been dismissed for challenging management.